1. Field of the Invention
The invention relates to a digital optical switch including a first section, called input section, having a first input waveguide and a second input waveguide, these waveguides converging toward each other to, and engaging each other at, the point of their coupling range, and a second section, called output section, having a third output waveguide and a fourth output waveguide, these output waveguides diverging from each other from this point to a point outside of their coupling range, the waveguides of the second section connecting to those of the first section, and means for controlledly influencing the state of light propagation in accordance with the adiabatic mode evolution.
Optical switches are attractive components for the connection of broad-banded optical signals in optically transparent networks, for the protection of control circuitry for by-passing a defective system or cable, and for spatial switches in transparent optical network nodes. Switching of a TV cable in a local area network (LAN), of broad-band optical ports of a computer or of optical signals in sensors and automobiles constitute further applications in the fields of telecommunications, micro system technology and automotive vehicles. These different applications require distinct switching parameters, such as low cross-talk and low switching power, independence of polarization and wavelength immunity.
2. Prior Art
The state of the art from which the invention is proceeding, may be taken from U.S. Pat. No. 4,775,207 or Appl. Phys. Lett. 51 (16), Oct. 19, 1987, pp. 1230-1232, and from European Patent 0 457 406.
The first-mentioned publications relate to a digital optical switch provided with waveguides based upon LiNbO.sub.3 and disposed in an X-shaped arrangement, the width of the two converging waveguides at their input section being different. The output waveguides, commencing at their point of engagement, are embraced by electrodes. The two input waveguides in the input section and, in the output section, the two output waveguides extending from the point of engagement, are disposed relative to each other at an angle .THETA. where .THETA.&lt;&lt;.DELTA..beta./.gamma., .DELTA..beta. being the average difference of the propagation constants of the two symmetric modes and .gamma. being the transverse wave constant in the vicinity of the waveguides. In this four-gated digital electro-optical switch, the switching operation is based upon the adiabatic mode evolution with reference to asymmetric width of the input waveguides. In this manner only a predetermined mode can controlledly propagate in the waveguides at an appropriate switching of the output waveguides.
In European Patent 0,457,406, there is described a digital optical switch in which the input waveguides in the input section are of asymmetric shape and in which the output waveguides in the output section are asymmetric or symmetric relative to each other and may be electrically switched, the asymmetry in the shape of the waveguides in the input and output sections being realized by a straight and a curved waveguide converging toward or diverging from each other. The purpose of the curved shape of the waveguides converging toward each other in the input section and diverging from each other in the output section is to reduce the length of the component relative to the previously described digital optical switch (DOS).
In the arrangements of 2.times.2 DOS's thus far described by the prior art the asymmetry required for the principle of adiabatic mode evolution has been realized, at least in part, in the input and/or output section thereof during the its fabrication. Adiabatic light propagation is possible only at small changes of the waveguide parameters so that the optical energy impinging upon the switch in a given basic mode, is maintained in this mode, i.e., no mode transformation occurs. If, therefore, optical energy of a low mode order impinges upon the switch, the light will be conducted through the output waveguide of the higher refractive index, resulting in a higher extinction ratio if the optical energy propagates essentially adiabatically in the switch.
Furthermore, digital optical switches are known which are constituted by 1.times.2-Y-branches. Thus, there was a report about a digital thermo-optical 1.times.2 switch made of polymer (see: Proc. 21st Eur. Conf. on Opt. Comm., pp 1063-1065) in which the waveguides are buried, the output branches include an angle of 0.12.degree., and heating electrodes are arranged completely to cover both branches. Upon heating an output branch light will be conducted into the unheated branch. An extinction coefficient better than 20 dB at a switching power between 130 mW and 230 mW was measured in the unheated branch; at about 180 mW the extinction coefficient reached a value of 27 dB.
In another report from ECOC'95--Brussels a 1.times.8 DOS is described for the first time which is constructed of three cascades of 1.times.2 switches (see: Proc. 21st Eur. Conf. on Opt. Comm., pp. 1059-1062. This solution, too, makes use of the thermo-optic effect in polymeric waveguides, which at a low switching power is capable of effecting a large change of the refractive index and, hence, a controlled conduction of the mode.